This invention relates to a liquid crystal display device and a method of manufacturing the same, more particularly to an improved construction of a filling port for filling a liquid crystal into the cell and a method of sealing the filling port.
A conventional liquid crystal display device comprises a sealed casing or a cell, opposed electrodes in the cell and a liquid crystal interposed between the opposing electrodes. When an operating voltage is impressed across the opposing electrodes, the optical characteristic of the liquid crystal varies at the portion impressed with the voltage thus displaying a desired digit, letter or symbol. Such liquid crystal display devices are now extensively used in table type electronic computers, wrist watches or the like. The cell comprises a pair of parallel transparent insulating substrates made of glass, for example, with the peripheries of the substrates spaced and sealed by a peripheral sealing member privoded with a filling port for the liquid crystal. However, the sealing of the filling port after the liquid crystal has been filled into the space defined by the insulating substrates and the peripheral sealing member involves a difficult problem. More particularly, the construction of the filling port sealing member must satisfy the following requirements. Since the liquid crystal is affected by moisture, the port sealing member should not permeate moisture. Secondly, since the liquid crystal has a large chemical affinity toward other substances, the port sealing member must be stable. Thirdly, since the liquid crystal decomposes at a high temperature the port sealing member must be applied at a low temperature. Fourthly, air bubbles should not be entrained in the cell. Fifthly, the port sealing member should have a large mechanical strength. Sixthly, the port sealing member should have a small size.
In the prior art liquid crystal display device, an organic substance has been used to seal the filling port. However, an organic substance can not fulfill the first requirement, that is the moisture impervious property. For this reason, it is desirable to use an inorganic substance as the filling port sealing agent. However, where glass, typically frit glass, is used as the filling port sealing agent it can not satisfy the requirements that the filling port sealing agent should withstand to high temperatures and that air bubbles should not be trapped in the cell. Accordingly, in recent years it has been proposed to use a solder as the filling port sealing agent. In order to assure a better bonding of the solder, a thin metal film acting as a prime coating and capable of manifesting a large bonding force to the solder is firstly formed about the periphery of the filling port extending through the sides of the insulating substrates and that of the peripheral sealing member. Although various metals and various methods of forming a thin film thereof may be used, where the substrates are made of glass, a nichrome layer and a thin metal layer overlying the same are formed by vacuum deposition technique and then the solder is applied to the metal layer. As will be described later with reference to the accompanying drawings, if the metal layer were formed on an arbitrary area and an arbitrary amount of the solder is applied onto the metal layer, the bonding strength of the filling port sealing agent would not be sufficient. For example, where an arbitrary area of the metal film and an arbitrary amount of the solder were used, when 40 environment tests are repeated at a temperature of -25.degree. C to +25.degree. C and at a relative humidity of 90%, due to the difference between the thermal expansion coefficients of the solder which may be considered as a rigid body and the insulating substrates, cracks are formed in the insulating substrates at portions thereof near the filling port, and the cracks grow as the number of tests is increased. For this reason, in the prior art liquid crystal display device, it has been inevitable to cause cracks in the substrates even when the solder is firmly bonded to the metal layer. Accordingly, the liquid crystal leaks to the outside of the cell thus making it difficult to maintain satisfactory sealing for the liquid crystal filling port.